Plating method

ABSTRACT

The present invention provides compositions and methods for immersion depositing highly adherent silver layers. The compositions contain silver ions, water and a carboxylic acid-substituted nitrogen-containing heterocyclic compound. Such compositions and methods are particularly useful in the manufacture of electronic devices.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the field of metalplating. In particular, the present invention relates to the field ofimmersion silver plating.

[0002] Immersion or displacement plating is an electroless platingprocess, but is given a separate classification in the art. In immersionplating, deposition is by displacement of an elemental metal from asubstrate by metal ions in a plating solution. In electroless platingdeposition takes place primarily by autocatalytic reduction of metalions from solution. Such electroless plating requires the presence of areducing agent.

[0003] Immersion plating does not employ an external electric currentbut rather is an electrochemical displacement reaction which is drivenby the position of the substrate metal in the electromotive seriesrelative to the metal to be deposited from solution. Plating occurs whenthe dissolved metal ions in a plating bath are displaced by a moreactive (less noble) metal that is contacted with the plating bath.

[0004] In the manufacture of printed wiring boards, solderable finishesare typically applied to printed wiring board substrates having padsand/or through holes exposed through a mask, such as a soldermask. Suchsolderable finishes are often applied by immersion plating aselectroless plating can also deposit metal on the surface of the mask,which is undesirable. As an immersion plating reaction is driven by thedifference in electrochemical potentials, plating will only occur atareas of exposed metal. However, there is a growing demand for moreenvironmentally acceptable alternatives to lead for use in printedwiring board manufacture. Thus, the use of lead and lead alloys inelectronic components faces an uncertain future.

[0005] Silver is a more environmentally acceptable alternative to leadand has been suggested for use as a solderable finish. As discussedabove, the preferred method of depositing such a solderable finish is byimmersion plating. For example, U.S. Pat. No. 5,955,141 (Souter et al.)discloses certain immersion silver plating baths suitable for depositinga layer of silver on a printed wiring board.

[0006] Limitations on the use of immersion plating exist in printedwiring board manufacture. Such limitations include relatively slowplating rates and limited deposit thicknesses, which are due to theself-limiting nature of immersion plating, i.e. as the metal depositbuilds, it tends to mask the underlying base metal, thereby preventingfurther displacement. These problems have conventionally been addressedusing a broad range of additives in the immersion plating bath, such asrate enhancers. However, such additives may adversely affect otherimportant characteristics of the deposit, such as adhesion and deposituniformity.

[0007] Therefore, there is a need for a method of improving the adhesionof immersion plated silver deposits. There is a further need for methodsthat do not adversely affect other important characteristics of thesilver deposit obtained from immersion plating baths.

[0008] Picolinic acid has been used in a tarnish remover composition forsilver and copper, see Indian Patent No 163677. Zhuang et al., HuaxueXuebao, 1985, vol. 43, no. 2, pp 120-125, disclose the electrodepositionof silver from a plating bath containing silver nitrate, ammonia andpyridine carboxylic acid. This article does not disclose the use ofpicolinic acid in an immersion silver plating bath.

SUMMARY OF THE INVENTION

[0009] It has been surprisingly found that an electroless, particularlyan immersion, silver plating bath containing as a complexing agent oneor more carboxylic acid-substituted nitrogen-containing heterocycliccompounds provides a subsequently immersion deposited layer of silverhaving improved adhesion as compared to silver deposits obtained fromconventional immersion deposition methods without such a complexingagent. In addition, bright silver deposits are obtained using theseplating baths.

[0010] The present invention provides a method of depositing a layer ofsilver on a substrate including the step of contacting a substratehaving a layer of a metal that is less electropositive than silver withan immersion silver plating bath including one or more sources of silverions, water and one or more carboxylic acid-substitutednitrogen-containing heterocyclic compounds, wherein the silver platingbath has a pH of less than or equal to 4.

[0011] Also provided by the present invention is a method ofmanufacturing a printed wiring board including the step of contacting aprinted wiring board substrate having a layer of a metal that is lesselectropositive than silver with an immersion silver plating bathincluding one or more sources of silver ions, water and one or morecarboxylic acid-substituted nitrogen-containing heterocyclic compounds,wherein the silver plating bath has a pH of less than or equal to 4.

[0012] The present invention further provides an immersion silverplating bath including one or more sources of silver ions, water and oneor more carboxylic acid-substituted nitrogen-containing heterocycliccompounds. Such compositions are typically free of ammonia and free ofcyanide. The present plating baths may further contain one or morethickness control agents.

DETAILED DESCRIPTION OF THE INVENTION

[0013] As used throughout this specification, the followingabbreviations shall have the following meanings, unless the contextclearly indicates otherwise: ° C.=degrees centigrade;ca.=circa=approximately; g=gram; L=liter; g/L=grams per liter;mL=milliliters; wt %=percent by weight; DI=deionized; cm=centimeters;μin.=microinches; and μm=microns=micrometers (1 μin.=0.0254 μm).

[0014] The terms “printed circuit board” and “printed wiring board” areused interchangeably throughout this specification. As used throughoutthis specification, the term “complexing agent” includes ligands andchelating agents. Unless otherwise noted, all amounts are percent byweight and all ratios are by weight. All numerical ranges are inclusiveand combinable in any order, except where it is clear that suchnumerical ranges are constrained to add up to 100%.

[0015] The present invention provides a method of improving the adhesionof a layer of silver deposited from an immersion plating bath includingthe step of contacting a metal that is less electropositive than silverwith a bath containing one or more sources of silver ions, water one ormore carboxylic acid-substituted nitrogen-containing heterocycliccompounds, wherein the silver plating bath has a pH of less than orequal to 4. The present invention provides a layer of silver depositedfrom an immersion plating bath having increased adhesion as compared toa layer of silver deposited using a conventional immersion silverplating bath. Thus, more adherent silver deposits are obtained, whichare less likely to flake or abrade off than conventional immersionsilver deposits. The increased adhesion of silver deposits obtainedaccording to the present invention also provides better surface mountconnections between a printed wiring board and a surface mount device.In addition, the silver deposits obtained from the present plating bathsare bright.

[0016] Any water soluble silver salt may be used as the source of silverions in the present plating baths. Suitable silver salts include, butare not limited to, silver nitrate, silver acetate, silver sulfate,silver lactate, and silver formate. Typically, the source of silver ionsis silver nitrate. Mixtures of silver salts may also be used. The one ormore sources of silver ions are typically present in an amountsufficient to provide silver ions in solution in a concentration of from0.06 to 32 g/L, more typically from 0.1 to 25 g/L and still moretypically from 0.5 to 15 g/L.

[0017] Carboxylic acid-substituted nitrogen-containing heterocycliccompounds function as both complexing agents and adhesion promoters. By“carboxylic acid-substituted nitrogen-containing hetrocylic compound” ismeant any nitrogen-containing heterocyclic moiety having one or more ofits hydrogens replaced by one or more carboxylic acid groups (—CO₂H). Awide variety of nitrogen-containing heterocyclic moieties may be used,such as, but not limited to, pyridine, piperidine, piperazine,morpholine, pyrrole, pyrrolidine, triazole, and imidazole. Suchheterocyclic compounds may be fused to another ring such asbenzotriazole, benzimidazole, quinoline or isoquinoline, or may befurther substituted, or both. Suitable substitutent groups includewithout limitation hydroxy, (C₁-C₁₀)alkyl, (C₁-C₁₀)alkoxy, and halo.Exemplary carboxylic acid-substituted nitrogen containing heterocycliccompounds include, but are not limited to, pyridine carboxylic acids,pyridine dicarboxylic acids, piperidine carboxylic acids, piperazinecarboxylic acids, and pyrrole carboxylic acids. The pyridine carboxylicacids are particularly useful, such as picolinic acid, quonolinic acid,nicotinic acid and fusaric acid. Such compounds are generallycommercially available, such as from Sigma-Aldrich (Milwaukee, Wis.), ormay be prepared by methods known in the literature. The carboxylicacid-substituted nitrogen-containing compound may be used in a varietyof concentrations but is typically present in the immersion silverplating baths either in stoichiometrically equivalent amounts (based onthe amount of silver ion) or in a stoichiometric excess so that all thesilver ions may be complexed. The term “stoichiometric” as used hereinrefers to equimolar. Preferably, the one or more complexing agents arepresent in a higher molar concentration than the silver ions. The molarratio of the complexing agent to silver ions is generally ≧1:1, andtypically ≧1.2:1, more typically ≧2.0:1, and still more typically≧3.0:1. In general, the total amount of the one or more complexingagents is typically from 0.1 to 250 g/L, and particularly from 15 to 220g/L, more particularly from 25 to 200 g/L, and still more particularlyfrom 40 to 150 g/L.

[0018] It will be appreciated by those skilled in the art that more thanone carboxylic acid-substituted nitrogen-containing heterocycliccompound may be used. One or more other complexing agents, i.e. anycomplexing agent that is not a carboxylic acid-substitutednitrogen-containing heterocyclic compound, may also be added to thepresent plating baths.

[0019] The pH of the present plating baths is typically less than orequal to 4. Baths having higher pH values may be used, however, theresulting silver deposit may be significantly thinner. Preferably, thepH of the bath is <4. A particularly useful pH range is from 3 to 4.Other useful plating baths have a pH<3, such as a pH in the range of 2to 3. A buffering agent may be added to maintain the pH of the bath atthe desired value. Any compatible acids or bases may be used as thebuffering agents and may be organic or inorganic. By “compatible” acidsor bases it is meant that the acids or bases do not result inprecipitation of the silver ions and/or complexing agents(s) fromsolution, when such acids or bases are used in amounts sufficient tobuffer the pH. Exemplary buffering agents include, without limitation,citric acid, tartaric acid, nitric acid, acetic acid and phosphoricacid.

[0020] The immersion silver plating baths may optionally contain one ormore additional components. Suitable additional components include, butare not limited to, surfactants or wetting agents, anti-tarnish agentsfor silver, oxidation inhibitors, levelers, grain refiners, thicknesscontrollers, defoamers, dyes, and the like. When surfactants are used inthe present immersion baths, they are typically used in an amount offrom 0.02 to 100 g/L, more typically from 0.1 to 25 g/L, and even moretypically from 1 to 15 g/L. Such surfactants may be anionic, cationic,nonionic or amphoteric. The choice of surfactant will depend upon theparticular immersion silver plating bath used. When surfactants arepresent, nonionic surfactants are preferred.

[0021] A wide variety of anti-tarnish agents for silver may be used,such as triazoles, tetrazoles, and imidazoles. Such anti-tarnish agentsare well known to those skilled in the art. The anti-tarnish agents maybe used in an amount from 0.001 to 50 g/L, more typically from 0.005 to25 g/L, and still more typically from 0.01 to 10 g/L.

[0022] Grain refiners are typically added to improve the appearance ofthe silver deposit. Exemplary grain refiners include, withoutlimitation, (C₁-C₆)alcohols and polyalkylene glycols such aspolyethylene glycol. Such grain refiners are typically used in an amountfrom 0.02 to 200 g/L, and more typically from 0.05 to 100 g/L.

[0023] Deposit thickness may be controlled by the addition of one ormore thickness control agents. Any agent suitable for controlling thethickness of an immersion silver deposit may be added to the presentplating baths. Such thickness control agents include rate enhancers andrate inhibitors. Suitable rate enhancers include, but are not limitedto: amino acids such as glycine, DL-lysine, β-alanine, glutamic acid andDL-aspartic acid; sulfur-containing compounds such as mercaptodiaceticacid; hydroxy-substituted aromatic compounds such as5-methoxyresorcinol; and azoles such as imidazole, 2-imidazolidone and2,4-imidazolidinedione. Suitable rate inhibitors include, withoutlimitation: azoles such as imidazole, 4-phenylimidazole,1,2,4-triazolo[1,5-α]pyrimidine, 4-amino-1,2,4-triazole and2-imidazolidone; and hydroxy-substituted aromatic compounds such as5-methoxyresorcinol. Suitable thickness control agents are the azoles,amino acids and the sulfur-containing compounds. Particularly usefulthickness control agents are glycine, DL-lysine, mercaptodiacetic acidand 1,2,4-triazolo[1,5-α]pyrimidine. It will be appreciated by thoseskilled in the art that mixtures of thickness control agents can beused. Such mixtures can contain two or more rate enhancers, two or morerate inhibitors or a combination of one or more rate inhibitors with oneor more rate enhancers. Certain thickness control agents can function asa rate enhancer at one pH and as a rate inhibitor at another pH.Likewise, certain thickness control agents can function as a rateenhancer when present in the plating bath at one concentration andfunction as a rate inhibitor when present at a different concentration.Such behavior is well known to those skilled in the art or can bedetermined by simple experimentation. Suitable thickness control agentsare generally commercially available such as from Sigma-Aldrich.

[0024] Typically, the thickness control agents may be present in theimmersion silver plating baths in an amount from 0.01 to 10 g/L, moretypically from 0.1 to 5/L and still more typically from 0.5 to 3 g/L.

[0025] Preferably, the immersion silver baths are free of reducingagents capable of reducing silver ions. It is further preferred that theimmersion silver baths are free of cyanide ions, ammonia and ammoniumions (i.e. NH₄ ⁺). More preferably, the present plating baths containonly one or more carboxylic acid-substituent nitrogen-containingheterocyclic compounds as complexing agents, although one or more othercomplexing agents may be used in combination with thenitrogen-containing heterocyclic compound depending upon the desiredapplication.

[0026] The immersion silver baths are typically prepared by combiningthe above ingredients in any order. Preferably, the baths are preparedby forming a solution of complexing agent in water and adding the sourceof silver ions to this solution. The optional ingredients may becombined with the solution in any order.

[0027] The immersion silver plating baths may be agitated. Any form ofagitation may be used. Suitable agitation includes, but is not limitedto, stirring, shaking, swirling, aeration, sonication and the like.Stirring may be accomplished by any suitable means, such as with anoverhead stirrer, paddle stirrer or stirring bar system. Shaking may beaccomplished in a variety of ways, such as by moving the substrate to besilver plated back and forth or side to side within the plating bath.Aeration may be accomplished by bubbling or sparging a gas into theplating bath, or by means of jet spray. Sparging may be accomplished bybubbling gas into the bath through a fritted means, such as a tubehaving a frit composed of glass, poly(tetrafluoroethylene) or otherinert material. Any gas may be used such as air, oxygen or an inert gas,and preferably air. In a particular embodiment, aeration is used toagitate the bath. Swirling may be accomplished by moving the bath or thesubstrate to be plated in a substantially circular motion. It will beappreciated by those skilled in the art that a combination of agitationmethods may be employed, such as stirring with aeration.

[0028] Silver is deposited on any substrate having a metal layer that isless electropositive than silver by contacting the metal layer with thepresent plating baths. Such contact may be by a variety of means, suchas dipping, spraying, flood coating, and the like. A particularlysuitable method of contacting with the present compositions is sprayingin a flood mode.

[0029] Suitable metals that are less electropositive than silverinclude, but are not limited to, zinc, iron, tin, nickel, lead, copperor alloys containing one or more of these metals. Particularly usefulmetals are tin, copper or alloys of tin or copper. A particularlysuitable alloy is tin-copper. In an alternate embodiment, such metal mayitself be immersion deposited on a suitable metal underlayer prior todepositing an immersion silver layer according to the present invention.For example, the metal may be tin, such tin deposit being firstdeposited, such as by immersion, electroless or electrolytic deposition,on copper.

[0030] Prior to silver plating, the metal to be plated is typicallycleaned. Cleaning removes oxides and organic contaminants from the metalsurface as well as resist residues that may remain from incompletedevelopment of photoresists as well as soldermask residues from coppersurfaces. Such cleaning may be by any suitable cleaning processes and/orproducts. For example, when the metal layer is copper or a copper alloy,it is preferred that the metal layer is cleaned with an acidic cleaningcomposition. Such cleaning procedures are well within the ability of oneskilled in the art. After cleaning, the substrate is typically rinsed,such as with water, and optionally dried. The metal layer may bemicroetched either before or after the cleaning step, and typicallyafter the cleaning step. Such microetching is accomplished by contactingthe metal layer on the substrate with a microetching composition, suchas sulfuric acid/hydrogen peroxide or an alkali metal persulfate such assodium or potassium persulfate. When such a microetching step is used,the metal layer may then optionally be rinsed with water or an acid,such as with sulfuric acid, to remove any residues from the cleaningand/or microetching steps.

[0031] Optionally, the metal to be silver plated may be contacted with apretreatment composition after cleaning and before contact with thesilver plating bath. Any suitable pretreatment composition may be used.Preferred pretreatment compositions include one or more azole compounds,water and a chelating agent.

[0032] A wide variety of azole compounds may suitably be used in thepretreatment compositions. Suitable azoles include, but are not limitedto, triazoles, benzotriazoles, tetrazoles, imidazoles, benzimidazoles,indazoles and mixtures thereof. Such azoles may optionally besubstituted.

[0033] Particularly useful azole compounds are benzotriazole,substituted benzotriazole, imidazole and substituted imidazole, and morespecifically benzotriazole, imidazole, (C₁-C₁₆)alkylimidazole, andarylimidazole. Phenylimidazole is the preferred arylimidazole. Exemplary(C₁-C₁₆)alkylimidazoles include methylimidazole, ethylimidazole,propylimidazole, hexylimidazole, decylimidazole and undecylimidazole.Such azole compounds are generally commercially available, such as fromSigma-Aldrich (Milwaukee, Wis.) and may be used without furtherpurification. The azole compounds may be used in the pretreatmentcompositions in a wide range of amounts. Typically, the azole compoundis used in an amount of from 0.005 to 50 g/L, preferably from 0.005 to20 g/L, and more preferably from 0.01 to 15 g/L. The specific amount ofazole compound depends upon the particular azole chosen and itssolubility in the pretreatment composition.

[0034] Such pretreatment compositions may be alkaline or acidic and havea pH of from 1 to 14. The pH of the pretreatment compositions may bevaried in order to increase the solubility of the azole compound. Forexample, the solubility of hydroxybenzotriazole can be increased byincreasing the pH of the pretreatment composition.

[0035] A wide variety of organic and inorganic acids or organic andinorganic bases can be used to adjust the pH of the pretreatmentcompositions. Suitable inorganic acids include, but are not limited to:hydrochloric acid, hydrofluoric acid, fluoroboric acid, hydroiodic acid,periodic acid, phosphoric acid, sulfuric acid, and nitric acid. Suitableorganic acids include, but are not limited to: alkylsulfonic acids, andarylsulfonic acids. Suitable inorganic bases include, but are notlimited to, alkali metal hydroxides, alkali metal carbonates, andammonium hydroxide. Suitable organic bases include, but are not limitedto, tetraalkylammonium hydroxides, and amines. The acids and/or basesare typically present in the pretreatment compositions in an amountsufficient to provide the desired pH.

[0036] Preferably, the pretreatment compositions further include one ormore chelating agents. Such chelating agents may be monodentate ligands,such as ammonia, cyanide, and pyridine, or multidentate ligands.Preferably, the chelating agents are the same as those used in thesubsequent immersion silver plating bath. Other additional componentsmay optionally be used in the pretreatment compositions. Such additionalcomponents include, but are not limited to, surfactants, metal ions, andthe like. The surfactants may be anionic, cationic, nonionic oramphoteric. When a surfactant is used in the present pretreatmentcompositions, it is typically present in an amount of at least 0.001 wt%, more typically at least 0.005 wt %, and still more typically at least0.01 wt %. Mixtures of surfactants may suitably be used.

[0037] When metal ions are present in the pretreatment compositions, itis preferred that they are the metal on which silver is to be deposited.For example, when silver is to be immersion deposited on copper, it ispreferred that any metal ions present in the pretreatment compositionare copper ions.

[0038] The amount of metal ions added to the pretreatment compositiondepends upon the particular metal that is less electropositive thansilver, the particular azole compound used and the pH of thepretreatment composition. For example, when copper ions are present inthe pretreatment composition, they are typically present in an amount upto 1 g/L, and more typically up to 0.05 g/L.

[0039] In general, the metal is contacted with the pretreatmentcomposition for a period of time sufficient to increase the adhesion ofa subsequently deposited layer of silver by immersion plating. Suchperiod of time depends upon the particular metal and pretreatmentcomposition used. Typically, a contact time of 1 second to 15 minutes issufficient. Other suitable ranges of contact times include, withoutlimitation, 5 seconds to 10 minutes, or 10 seconds to 5 minutes.

[0040] After the metal has been contacted with the pretreatmentcomposition, it is optionally rinsed, such as with water, and thenoptionally dried. Such a rinsing step is preferred.

[0041] Silver is deposited according to the present invention bycontacting a metal that is less electropositive than silver with thepresent immersion silver plating baths. Such contact may be by dipping,spraying, flood coating, and the like. When used in vertical platingequipment, the substrate is typically dipped in the present silverplating bath. When used in horizontal plating equipment, the substrateis typically contacted with the present silver plating bath by sprayingor flooding.

[0042] The contact time of the metal with the immersion silver platingbath is that amount sufficient to deposit a desired thickness of silver.Typically, the contact time is from 10 seconds to 15 minutes, moretypically from 20 seconds to 15 minutes, and still more typically from30 seconds to 12 minutes.

[0043] The present immersion silver plating baths may be used at avariety of temperatures. Suitable temperatures include those in therange of from 10° to 70° C. Other suitable temperature ranges are from15 to 60° C., and from 200 to 55° C.

[0044] The silver deposit typically has a thickness of 35 μin. (0.9 μm)or less, more typically 30 μin. (0.76 μm) or less, and even moretypically 25 μin. (0.64 μm) or less. Following deposition, the silverlayer may be rinsed such as with water. The silver layer may optionallybe dried prior to subsequent processing steps.

[0045] In general, the metal that is less electropositive than silver isa metal layer on a substrate. A wide variety of substrates having alayer of a metal that is less electropositive than silver may be platedaccording to the present invention. Suitable substrates include, but arenot limited to, jewelry, decorative objects, object d'art, semiconductorpackaging, lead frames, solder bumps, metal powder, metal foils such ascopper foil, and printed wiring board substrates. The present inventionis particularly suited for depositing a solderable silver finish on aprinted wiring board. Further, the present invention provides a methodof manufacturing a printed wiring board including the steps ofcontacting a metal that is less electropositive than silver with animmersion silver plating bath containing one or more sources of silverions, water and one or more carboxylic acid-substitutednitrogen-containing heterocyclic compounds to provide a layer of silver,wherein the bath has a pH of ≦4. Such a silver layer provides asolderable finish on the printed wiring board. Such solderable finishesare typically applied to a printed wiring board substrate having pads,through holes and a mask, such as a soldermask. In such a printed wiringboard substrate, the exposed pads and through holes generally contain alayer of copper.

[0046] In yet another embodiment, the present invention is suitable forproviding a printed wiring board substrate having exposed pads and/orthrough holes including a tin-silver alloy as the solderable finish.Thus, the present invention further provides a method for manufacturinga printed wiring board including the steps of: a) contacting a printedwiring board substrate having pads, through holes, soldermask and alayer of a metal that is less electropositive than tin with an immersionplating bath including a source of tin ions, water and a complexingagent, to form a tin deposit on the metal; b) then contacting the tinplated printed wiring board substrate with an immersion silver platingbath including one or more sources of silver ions, water and one or morecarboxylic acid-substituted nitrogen-containing heterocyclic compounds,to form an immersion silver deposit on the tin deposit; and c) heatingthe silver-tin deposit to form a tin-silver alloy.

[0047] A wide variety of post-treatments may be used to treat the silverlayer deposited according to the present invention. For example, it iswell known that silver, such as in silver films, tarnishes uponprolonged exposure to air. Thus, in certain applications it is desirableto contact the freshly deposited silver layer with a tarnish inhibitoror anti-tarnish agent. Such silver tarnish inhibitors are well-known tothose skilled in the art and include those described above. The silverdeposit may be contacted with the tarnish inhibitor by any suitablemeans, such as, but not limited to, dipping, spraying, and floodcoating. The use of a tarnish inhibitor subsequent to plating is notrequired, but may optionally be used. Other suitable post-treatments mayalso be advantageously used.

[0048] The present invention is particularly suitable for use in themanufacture of a wide variety of electronic devices in addition toprinted wiring boards, such as lead frames, semiconductor packaging, andlead-free solder bumps on wafers, such as tin-silver andtin-copper-silver solders. The present invention is suitable for use invertical or horizontal plating equipment.

[0049] The following examples are expected to further illustrate variousaspects of the present invention, but are not intended to limit thescope of the invention in any aspect.

EXAMPLE 1

[0050] An immersion silver plating bath was prepared by combining 50 g/Lpicolinic acid and 1 g/L silver nitrate in water (final volume 1L). ThepH of the bath was adjusted to 3, The bath temperature was adjusted toca. 50° C.

EXAMPLE 2

[0051] The procedure of Example 1 was repeated, except that anotheradditive was added to the bath. These additives are listed in Table 1.The pH of these baths was again adjusted to 3. The bath temperature wasadjusted to ca. 50° C.

[0052] Copper panels (2×6 inches or 5×15 cm) were submerged in acommercially available acid cleaner to remove oxides and organicresidues from the copper surface, followed by rinsing with water. Thecopper panels were next contacted with a commercially available sulfuricacid/hydrogen peroxide-based microetching composition to produce optimumcopper surface uniformity and texture, followed by rinsing with water.After contact with the microetching composition, the copper panels werethen submerged in either the silver plating bath of Example 1 or theplating bath containing additional additives of this example for 10minutes.

[0053] The thickness of the resulting silver layer on the copper panelsfor each silver formulation was determined by X-ray fluorescence (“XRF”)spectroscopy for a number of points on the panel and the data arereported as thickness ranges in Table 1. The picolinic acid and additiveconcentrations and the corresponding effect on silver plate thickness,obtained from XRF measurements, are illustrated below.

[0054] The silver plated copper panels were also evaluated to determinethe adhesion of the silver layer. A 3×1 inch (7.6×2.5 cm) strip oftransparent adhesive tape, Scotch 610 brand available from the 3MCompany, Minneapolis, Minn., was applied to the surface of each of thesilver coated copper panels. The tape was then removed from each panel.Silver deposits having poor adhesion, were readily removed by the tapewere rated as “failed.” Silver deposits having good adhesion were notremoved by the tape were rated as “passed.” The adhesion results arealso reported in Table 1. TABLE 1 Immersion Amount of Thickness SilverPlating additive Range Tape Bath Additive (g/L) (μm) Appearance TestExample 1 — — 0.091-0.18  Bright Passed Example 2A L-Glutamic Acid 0.210.12-0.23 Bright Passed 0.3 0.13-0.20 Bright Passed 0.5 0.073-0.12 Bright Passed Example 2B Glycine 0.1 0.10-0.22 Bright Passed 0.40.17-0.27 Bright Passed 1.0 0.19-0.31 Bright Passed Example 2C DL-Lysine0.01 0.071-0.20  Bright Passed 0.05 0.12-0.24 Bright Passed 0.10.12-0.28 Bright Passed 0.5 0.38-0.52 Bright Passed 1.5 0.40-0.57 BrightPassed Example 2D Mercaptodiacetic Acid 0.1 0.19-0.27 Bright Passed 0.250.14-0.22 Bright Passed 0.30 0.26-0.42 Bright Passed Example 2EImidazole 0.25 0.053-0.12  Bright Passed 0.50 0.049-0.13  Bright PassedExample 2F 4-Phenylimidazole 0.15 0.30-0.45 Bright Passed 0.25 0.16-0.39Bright Passed 0.30 0.058-0.10  Bright Passed Example 2G 2-Imidazolidone0.3 0.10-0.23 Bright Passed 0.5 0.067-0.09  Bright Passed 1.00.076-0.11  Bright Passed Example 2H 1,2,4-Triazolo[1,5- .01 0.078-0.15 Bright Passed α]pyrimidine .05 0.061-0.11  Bright Passed 0.250.094-0.12  Bright Passed 0.1 0.082-0.13  Bright Passed Example 2I4-Amino-1,2,4-Triazole 0.31 0.070-0.11  Bright Passed Example 2J2-Imidazolidone 0.3 0.10-0.23 Bright Passed 0.5 0.067-0.089 BrightPassed Example 2K DL-Aspartic Acid 0.05 0.090-0.26  Bright Passed 0.100.10-0.17 Bright Passed 0.25 0.068-0.10  Bright Passed Example 2L5-Methoxyresorcinol 0.01 0.087-0.16  Bright Passed 0.02 0.13-0.27 BrightPassed 0.05 0.083-0.13  Bright Passed 0.1 0.088-0.12  Bright PassedExample 2M β-Alanine 0.3 0.12-0.26 Bright Passed 0.5 0.075-0.24  BrightPassed 1.0 0.11-0.21 Bright Passed Example 2N Hydantoin 0.3 0.22-0.32Bright Passed 0.5 0.13-0.24 Bright Passed 1.0 0.18-0.27 Bright Passed

[0055] From these data it can be seen that highly adherent, brightsilver deposits were obtained. EXAMPLE 3

[0056] The procedure of Example 1 was repeated, except that 0.01 and0.05 g/L 1,2,4-triazolo[1,5-α]pyrimidine was also added to thecompositions to provide Examples 3A and 3B. The pH of the baths wasadjusted to 3. In each case, the bath temperature was adjusted to ca.50° C.

EXAMPLE 4

[0057] The procedure of Example 3 was repeated, except that 0.5 g/Lglycine was added to each of the compositions to provide Examples 4A and4B. These additives are listed in Table 2. The pH of these baths wasagain adjusted to 3. The bath temperature was adjusted to ca. 50° C.

[0058] Copper panels (2×6 inches or 5×15 cm) were submerged in acommercially available acid cleaner to remove oxides and organicresidues from the copper surface, followed by rinsing with water. Thecopper panels were next contacted with a commercially available sulfuricacid/hydrogen peroxide-based microetching composition to produce optimumcopper surface uniformity and texture, followed by rinsing with water.After contact with the microetching composition, the copper panels werethen submerged in either the silver plating baths of Example 3 or theplating baths of this example for 10 minutes.

[0059] The thickness of the resulting silver layer on the copper panelsfor each silver formulation was determined according to the procedure ofExample 2 and the data are reported in Table 2. The picolinic acid andadditive concentrations and the corresponding effect on silver platethickness, obtained from XRF measurements, are illustrated below. Thesilver plated copper panels were also evaluated to determine theadhesion of the silver layer using the procedure of Example 2. Theadhesion results are also reported in Table 2. TABLE 2 Immersion1,2,4-Triazolo Thickness Silver [1,5-a]pyri- Glycine Range Appear- TapePlating Bath midine (g/L) (g/L) (μm) ance Test Example 3A 0.01 00.078-0.15  Bright Passed Example 4A 0.01 0.5 0.056-0.13  Bright PassedExample 3B 0.05 0 0.061-0.11  Bright Passed Example 4B 0.05 0.50.043-0.092 Bright Passed

[0060] The above data clearly show that highly adherent, bright silverdeposits were obtained.

EXAMPLE 5

[0061] The procedure of Example 1 was repeated, except that 0.02 and0.05 g/L 5-methoxyresorcinol was also added to the compositions toprovide Examples 5A and SB. The pH of the baths was adjusted to 3. Ineach case, the bath temperature was adjusted to ca. 50° C.

EXAMPLE 6

[0062] The procedure of Example 5 was repeated, except that 0.5 g/Lglycine was added to each of the compositions to provide Examples 6A and6B. These additives are listed in Table 3. The pH of these baths wasagain adjusted to 3. The bath temperature was adjusted to ca. 50° C.

[0063] Copper panels (2×6 inches or 5×15 cm) were cleaned andmicroetched according to the procedure of Example 2. After contact withthe microetching composition, the copper panels were then submerged ineither the silver plating baths of Example 5 or the plating baths ofthis example for 10 minutes.

[0064] The thickness of the resulting silver layer on the copper panelsfor each silver formulation was determined by XRF spectroscopy accordingto the procedure of Example 2 and the data are reported in Table 3. Thesilver plated copper panels were also evaluated to determine theadhesion of the silver layer according to the procedure of Example 2.The adhesion results are also reported in Table 3. TABLE 3 Immersion5-Methoxy- Thickness Silver Glycine resorcinol Range Appear- TapePlating Bath (g/L) (g/L) (μm) ance Test Example 5A 0 0.02  0.13-0.27Bright Passed Example 6A 0.5 0.02 0.091-0.25 Bright Passed Example 5B 00.05 0.057-0.11 Bright Passed Example 6B 0.5 0.05 0.060-0.13 BrightPassed

[0065] These data show that bright, highly adherent deposits wereobtained.

EXAMPLE 7

[0066] The procedure of Example 1 was repeated, except that 0.10 g/L1,2,4-triazolo[1,5-α]pyrimidine was also added to the composition toprovide Example 7. The pH of the bath was adjusted to 3. The bathtemperature was adjusted to ca. 50° C.

EXAMPLE 8

[0067] The procedure of Example 7 was repeated, except that anotheradditive was added to the bath. These additives are listed in Table 4.The pH of these baths was again adjusted to 3. The bath temperature wasadjusted to ca. 50° C.

[0068] Copper panels (2×6 inches or 5×15 cm) were cleaned andmicroetched according to the procedure of Example 2. The copper panelswere next contacted with a commercially available sulfuric acid/hydrogenperoxide-based microetching composition to produce optimum coppersurface uniformity and texture, followed by rinsing with water. Aftercontact with the microetching composition, the copper panels were thensubmerged in either the silver plating bath of Example 7 or the platingbath containing additional additives of this example for 10 minutes.

[0069] The thickness of the resulting silver layer on the copper panelsfor each silver formulation was determined by XRF spectroscopy accordingto the procedure of Example 2 and the data are reported in Table 4. Thesilver plated copper panels were also evaluated to determine theadhesion of the silver layer according to the procedure of Example 2.The adhesion results are also reported in Table 4. TABLE 4 ImmersionAmount of Silver Additive Thickness Tape Plating Bath Additive (g/L)Range (μm) Appearance Test Example 7 — — 0.082-0.13 Bright PassedExample 8A L-Glutamic Acid 0.11 0.083-0.12 Bright Passed Example 8BDL-Aspartic Acid 0.05 0.075-0.14 Bright Passed 0.10 0.081-0.11 BrightPassed Example 8C Mercaptodiacetic Acid 0.3  0.14-0.28 Bright PassedExample 8D 2-Imidazolidone 0.51 0.070-0.20 Bright Passed Example 8EDL-Lysine 0.01  0.10-0.14 Bright Passed 0.05  0.11-0.18 Bright Passed0.1  0.15-0.22 Bright Passed 1.0  0.17-0.36 Bright Passed Example 8Fβ-Alanine 0.1 0.082-0.10 Bright Passed

[0070] These data clearly demonstrate that bright, highly adherentsilver deposits were obtained.

EXAMPLE 9

[0071] The procedure of Example 1 was repeated, except that 0.01 g/L1,2,4-triazolo[1,5-α]pyrimidine was also added to the composition toprovide Example 9. The pH of the bath was adjusted to 3. The bathtemperature was adjusted to ca. 50° C.

EXAMPLE 10

[0072] The procedure of Example 9 was repeated, except that anotheradditive was added to the bath. These additives are listed in Table 5.The pH of these baths was again adjusted to 3. The bath temperature wasadjusted to ca. 50° C.

[0073] Copper panels (2×6 inches or 5×15 cm) were cleaned andmicroetched according to the procedure of Example 2. After contact withthe microetching composition, the copper panels were then submerged ineither the silver plating baths of this example or the plating bathcontaining additional additives of Example 10 for 10 minutes.

[0074] The thickness of the resulting silver layer on the copper panelsfor each silver formulation was determined according to the procedure ofExample 2 and the data are reported in Table 5. The silver plated copperpanels were also evaluated to determine the adhesion of the silver layeraccording to the procedure of Example 2. The adhesion results are alsoreported in Table 5. TABLE 5 Immersion Amount of Silver AdditiveThickness Tape Plating Bath Additive (g/L) Range (μm) Appearance TestExample 9 — — 0.12-0.18 Bright Passed Example 10A L-Glutamic Acid 0.110.081-0.15  Bright Passed Example 10B DL-Aspartic Acid 0.10 0.11-0.17Bright Passed Example 10C Mercaptodiacetic Acid 0.10 0.16-0.30 BrightPassed

[0075] These data show that bright, highly adherent silver deposits wereobtained using the present immersion silver plating baths.

EXAMPLE 11

[0076] A number of silver plating baths were prepared by combining 1 gof silver nitrate and either 15 g, 25 g or 50 g of picolinic acid with 1L of DI water. The pH of each bath was then adjusted using acid or baseas appropriate to provide the desired pH. Each bath was then heated toca. 50° C.

[0077] Copper panels (2×6 inches or 5×15 cm) were cleaned andmicroetched according to the procedure of Example 2. Each copper panelwas then immersed in one of the plating baths. The copper panel werecontacted with the plating baths for a set time, after which the panelswere removed from the bath, rinsed with water and dried. The thicknessof the resulting silver deposit on each panel was then determinedaccording to the procedure of Example 2. The results are reported inTable 6. TABLE 6 15 g/L 25 g/L 50 g/L pH Picolonic Acid Picolonic AcidPicolonic Acid 3  0.17-0.28 μm  0.19-0.22 μm  0.13-0.18 μm 4  0.029-0.10μm 0.019-0.025 μm 0.029-0.033 μm 5 0.016-0.020 μm  0.023-0.33 μm0.022-0.029 μm 6 0.024-0.025 μm 0.029-0.045 μm 0.021-0.027 μm 70.013-0.018 μm 0.023-0.028 μm 0.023-0.028 μm

[0078] The above data clearly show that when picolinic acid is used asthe sole complexing agent, much less silver is deposited when the pH ofthe bath is greater than 4.

EXAMPLE 12

[0079] A procedure of Example 1 except that the picolinic acid isreplaced with one of the compounds in the amount shown in Table 7. TABLE7 Compound Amount (g/L) Nicotinic acid 45 Isonicotinic acid 40Quinolinic acid 55 Fusaric acid 35 Isonipecotic acid 60 Nipecotic acid65 2,6-pyridine dicarboxylic acid 58 Piperazine-2-carboxylic acid 30Pyrrole-2-carboxylic acid 35 Pipecolinic acid 50

EXAMPLE 13

[0080] Copper panels are cleaned and microetched according to theprocedure of Example 2, and are then to be contacted with the platingbaths of Example 12, to deposit a layer of silver.

What is claimed is:
 1. A method of depositing a layer of silver on asubstrate comprising the step of contacting a substrate having a layerof a metal that is less electropositive than silver with an immersionsilver plating bath comprising one or more sources of silver ions, waterand one or more carboxylic acid-substituted nitrogen-containingheterocyclic compounds, wherein the bath has a pH of less than or equalto
 4. 2. The method of claim 1 wherein the substrate is a printed wiringboard substrate.
 3. The method of claim 1 wherein the metal that is lesselectropositive than silver is chosen from zinc, iron, tin, nickel,lead, copper or alloys of zinc, iron, tin, nickel, lead and copper. 4.The method of claim 1 wherein the carboxylic acid-substitutednitrogen-containing heterocyclic compounds is chosen from picolinicacid, quinolinic acid, nicotinic acid, isonicotinic acid, fusaric acid,isonipecotic acid, nipecotic acid, 2,6-pyridine dicarboxylic acid,piperazine-2-carboxylic acid, pyrrole-2-carboxylic acid and piperolinicacid.
 5. The method of claim 1 wherein the carboxylic acid-substitutednitrogen-containing heterocyclic compound comprises anitrogen-containing heterocyclic moiety chosen from pyridine,piperidine, piperazine, pyrrole, morpholine, pyrrolidine, triazole, andimidazole.
 6. A method of manufacturing a printed wiring boardcomprising the step of contacting a printed wiring board substratehaving a layer of a metal that is less electropositive than silver withan immersion silver plating bath comprising one or more sources ofsilver ions, water and one or more carboxylic acid-substitutednitrogen-containing heterocyclic compounds, wherein the silver platingbath has a pH of less than or equal to
 4. 7. An immersion silver platingbath comprising one or more sources of silver ions, water and one ormore carboxylic acid-substituted nitrogen-containing heterocycliccompounds, wherein the silver plating bath has a pH of less than orequal to
 4. 8. The immersion silver plating bath of claim 7 wherein thebath is free of cyanide ions, ammonia and ammonium ions.
 9. Theimmersion silver plating bath of claim 7 wherein the carboxylicacid-substituted nitrogen-containing heterocyclic compound is chosenfrom picolinic acid, quinolinic acid, nicotinic acid, isonicotinic acid,fusaric acid, isonipecotic acid, nipecotic acid, 2,6-pyridinedicarboxylic acid, piperazine-2-carboxylic acid, pyrrole-2-carboxylicacid and piperolinic acid.
 10. The immersion silver plating bath ofclaim 7 wherein the carboxylic acid-substituted nitrogen-containingheterocyclic compound comprises a nitrogen-containing heterocyclicmoiety selected chosen from pyridine, piperidine, piperazine, pyrrole,morpholine, pyrrolidine, triazole, and imidazole
 11. The immersionsilver plating bath of claim 7 further comprising one or more thicknesscontrol agents.
 12. The immersion silver plating bath of claim 11wherein the thickness control agent is chosen from azoles, amino acids,hydroxy-substituted aromatic compounds, sulfur-containing compounds andhydantoins.